The present invention is related to a material suitable for a substrate on which a semiconductor device is mounted and a method of manufacturing said substrate material. The new material is excellent in thermal conductivity capable of efficiently releasing heat generated in a semiconductor device and has a thermal expansion coefficient similar to that of the semiconductor device.
In case where a material to be used as a substrate for mounting a semiconductor device thereon has a thermal expansion coefficent different from that of the semiconductor device, defects such as cracking or splitting are likely to occur in the semiconductor device, the substrate material or the junction plane therebetween due to thermal stress. In addition, when the heat diffusion from the semiconductor device is insufficient, the semiconductor device is heated at a temperature exceeding its rated value, resulting in malfunction or troubles. In this regards, the substrate material is required to have a thermal expansion coefficient similar to that of the semiconductor device, and sufficient thermal conductivity to maintain the semiconductor device at a temperature below its rated value.
Various kinds of substrate materials to satisfy the above mentioned requirements have been proposed. For instance, metallic materials, e.g. W, Mo, Coval or Alloy #42, and ceramic materials, e.g. alumina or beryllia have been used as substrates for mounting a semiconductor device thereon. In the case where excellent thermal conductivity is especially requested, copper alloys have been used as substrate materials.
By the way, the technology for improving the function of semiconductor devices and integrating semiconductor devices has been remarkably developing day by day. For instance, semiconductor devices are made larger in scale, and with high density more functions are incorporated in one element. Accompanied with these trends, the amount of heat generated in a semiconductor being used becomes larger. As a consequence, a substrate for mounting a semiconductor device thereon is required to have a thermal expansion coefficient similar to those of a semiconductor device and a package material as well as excellent thermal conductivity to efficiently diffuse heat generated in the semiconductor device.
In order to satisfy these requirements, there is proposed to disposed a sintered body as an electrode between a Si device and a copper terminal plate by Japanese Patent Application Laid-Open 50-62776. The sintered body contains components such as Cu or Ag, which are excellent in thermal conductivity and other components such as W or Mo, which are excellent in heat resistance. A substrate material proposed by Japanese Patent Application Laid-Open 59-21032 is a sintered body manufactured by impregnating a porous W presintered body with copper.
As for these sintered bodies, the thermal expansion coefficient and thermal conductivity may be freely determined by changing the content of Cu or Ag. When there is used a sintered body containing a proper amount of Cu or Ag most suitable for the material of a semiconductor device to be mounted as well as the configuration and the size of a package, it is expected to obtain a substrate having excellent thermal conductivity and a thermal expansion coefficient similar to that of a semiconductor device to be mounted thereon.
A Cu-W composite material has been manufactured by a powder metallurgy process. When a composite material containing a small amount of copper is to be manufactured, an impregnation method is adopted to perform a uniformly dispersion of Cu into W matrix. According to the impregnation method, W powder is compressed and formed into a porous presintered body (hereinafter referred to as "skeleton"), and the W skeleton is then impregnated with molten copper.
According to the impregnation method, it is easy to obtain a product having high density. However, it is difficult to uniformly disperse Cu throughout the entire W skeleton. That is, there remain parts into which molten copper is undispersed or insufficiently dispersed. As a result, it is inevitable that defects are formed such as pinholes, in the inner parts of the sintered body and on the surface layer of the sintered body.
When the substrate material involving pinholes is coated with a Ni or Au plating layer, other defects, such as blisters or unplated parts are formed in the plating layer. Further, the adherence of the plating layer to the substrate material is poor, so that the plating layer is likely to peel off the surface of the substrate material. Consequently, the junction of the substrate and the semiconductor device mounted thereon is deteriorated, and the contact resistance between the substrate and the semiconductor device becomes larger. A sufficient contact to diffuse a sufficient amount of heat flux is not formed between the semiconductor device and the substrate. Accordingly, the semiconductor device is overheated during operation, resulting in malfunctions and troubles.